The human body is equipped to adapt to the varying needs of chemical and electrical stimulants during everyday activity including exercise. If the heart is functioning properly, the nervous system increases the heart rate through electrical stimulation and reduces peripheral resistance in response to exercise. Similarly, the endocrine glands regulate production of chemical hormones to meet the varying demands of the body during everyday activity. However, a large, increasing population of patients have implantable devices to compensate for various heart conduction disorders. With rate adaptive and exercise-responsive pacemakers being developed, the pacemaker has not only become a life-sustaining device for a significant number of people with cardiac conduction problems, but it has also become a device for improving the quality of life for these patients to lead a more normal existence. In addition, cardiovascular treatment devices such as implantable defibrillators and medication dispensers are available for providing treatment in emergency and other situations.
Several physiological parameters have been utilized to administer cardiovascular treatment. These parameters include, amongst others, nerve electrical activity, biochemical concentrations such as enzymes and glucose, blood pressure, blood and body temperature, oxygen saturation, metal ion concentration (pH), respiration, motion, etc. Among other uses, physiological parameters have been utilized to indicate pacing rates during exercise including pH, QT interval, respiratory rate, body motion, and the venous blood temperature in the right ventricle of the heart.
From the original fixed-rate cardiac pacemaker evolved the demand pacemaker. The demand pacemaker ceases to produce an electrical stimulus when a spontaneous heart beat is detected. The presence of a spontaneous heart beat is indicated by a normal QRS complex in the electrocardiogram. In addition to sensing the presence of electrical activity in the ventricle, sensing of atrial activity has also been used. In an attempt to provide sensing information, the nerves leading to the heart, in particular the sympathetic nerves, will provide information processed by the brain that naturally increases the heart rate. Unfortunately, current technology prohibits the use of a long-term nerve impulse transducer.
The pH of the blood has also been measured and used to control the rate of a cardiac pacemaker. However, pH transducers that are implantable for long periods of time are difficult to produce and therefore are not yet in common use.
In another prior art cardiac pacemaker, ambient body temperature is measured by a charging capacitor having a high temperature coefficient located within the pulse generator circuitry. However, since ambient body temperature does not vary appropriately as a function of muscle exertion, this device will not respond to a body's need for increased cardiac output due to muscular exertion.
In still yet another cardiac pacemaker, the nonambient blood temperature in the right ventricle of the heart along with the sensing of body motion are utilized to control the stimulation of the heart.
In yet another prior art cardiac pacer, the respiratory rate is used to vary the production of electronic pulses which are fed to a constant current source connected to the ventricle of the heart.
In still another cardiac pacemaker, the oxygen saturation of the blood is measured as a control variable for influencing the frequency of stimulation. Light conductor probes implanted in the heart determine the blood oxygen saturation therein.
In a programmable tachycardia pacer, dual functions of demand pacing as well as standby tachycardia break-up are performed. A command parameter control circuit is used for programmably controlling the parameters of the pacer operation as well as of the tachycardia recognition and response.
In a self-contained artificial heart, the pulse rate and the stroke rate vary in response to blood pressure. Variations in blood pressure are detected by means of a pressure sensitive transistor, thereby varying the rate of pumping of blood in response to blood pressure.
As indicated from the above-described cardiovascular treatment devices, any number of physiological parameters may be utilized for providing electrical stimulation or chemical treatment of the patient. Often, sensors for each of these physiological parameters are implanted within the patient to indicate a particular measure or level of the physiological parameter to control the stimulation rate or treatment of the patient.
With respect to sensing nonambient venous blood temperature as disclosed in U.S. Pat. Nos. 4,436,092 and 4,543,954 of the present inventor, a thermistor along with pacing electrodes are positioned within the right ventricle of the heart for sensing the nonambient temperature of blood returning to the right ventricle of the heart. A separate lead extending from the pacemaker housing connects to the thermistor for sensing the nonambient blood temperature. The resistance of the thermistor varies as the temperature of the blood changes, thereby indicating changes in the exercise level of the patient. The changes in resistance of the thermistor are externally sensed and fed back to the control circuit of the pacemaker positioned within the pacemaker housing.
Similarly, a photoemitter and detector are positioned along with the pacing leads in the right ventricle of the heart to sense changes in oxygen saturation of the blood. Changes in the reflectance or density of the optical signal emitted from the photoemitter are sensed by the photodetector to indicate the level of oxygen saturation in the right ventricle of the heart.
In addition, pressure transducers are also positioned in the heart along with the pacing leads to indicate a measure or level of blood pressure changes for indicating stimulation rates.
A problem associated with all of these sensors is that they are placed external to the pacemaker container along with the electrical conductors leading thereto for sensing the physiological parameter. At a minimum, such sensors increase the bulk and complexity of the pacemaker lead leading to the heart. Sensors positioned external to the pacemaker housing and heart also present additional problems of fixation and migration. This is particularly true with respect to measuring respiratory rate or parameters associated with respiration.
Another problem with these externally placed sensors is the increased risk of failure and subsequent surgical intervention. Surgical intervention to remove and replace these externally positioned sensors involves lifethreatening risks and associated medical expense